Recently, as a market for mobile communication terminals, such as a mobile phone and a personal digital assistant (PDA), and Bluetooth products for facilitating ubiquitous communications is growing rapidly, high-frequency devices constituting them, such as a microwave filter, a duplexer, a resonator, and an integrated circuit board are required to become smaller and lighter, and to be stacked and surface-mounted.
Such high-frequency devices include dielectric ceramic materials. The dielectric ceramics for the high-frequency devices should have specific dielectric characteristics as follows.
First, in order to reduce the device size, the dielectric ceramics should have a high dielectric constant, ∈r. This is because a wavelength of the microwave in the dielectric ceramics is decreased in inverse proportion to the square root of the dielectric constant. However, a microwave transmission line provided to a board of radio frequency (RF)/microwave module should rather have a low dielectric constant so as to increase the speed.
Second, for a highly efficient operation, the dielectric ceramics should have a high quality factor (Q) within an operation frequency range. In other words, the dielectric ceramics should have a low dielectric loss, tan δ, which is a reciprocal of the quality factor. In general, the quality factor is evaluated based on the product of the quality factor and a corresponding resonance frequency, Q×f, or the dielectric loss, a reciprocal of the quality factor.
Third, for an accurate operation of the operation frequency, the dielectric ceramics should have a temperature coefficient factor (TCF) of the resonance frequency, τf, close to zero.
Meanwhile, a method for stacking high-frequency devices under development in recent times includes printing a conductive pattern on a green sheet of dielectric ceramics, stacking the printed green sheets, and then sintering them. This method allows lots of elements such as an inductor, a capacitor and a resistor to be integrated in a single module without additional lead wire. Accordingly, the package size can be reduced significantly.
However, the method requires that an internal conductor formed of silver (Ag) or copper (Cu) having excellent conductivity should be sintered together with the dielectric ceramics. Accordingly, a low temperature co-fired ceramics (LTCC) is demanded strongly. The LTCC can be sintered at a temperature lower than approximately 950° C., however, has a high quality factor and a low resonance frequency. However, most of the recently developed LTCC suffers from significantly deteriorated microwave dielectric characteristics, such as insufficient densification, low dielectric constant clue to the addition of sintering agents, lowered quality factor, increased temperature coefficient factor of the resonance frequency, and the like.
Furthermore, the typical LTCC is formed of ceramic materials having a composite structure including a glass matrix and an alumina (Al2O3) powder filler mixed thereto. However, this typical LTCC is reported to suffer from difficulty in controlling rheology during the ceramic slurry formation, ununiform glass composition, ununiform dispersion, and the like. Consequently, a glass-free (or non-glass) LTCC composition including no glass or minimum amount of glass is attracting considerable interests.